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We're a semver v1, so we can't remove exposed APIs unless we bump to
v2+. Bumping the major version to just rename two APIs seems a bit
overkill; keeping the old ones around and deprecating them is much
easier.

This partially reverts commit b8473079.

At least go-merkledag and go-graphsync still need a prototype chooser,
and go-ipld-prime-proto provides one, so this simplifies integration
with downstreams.

It's worth noting that we just add PBNode here, while
go-ipld-prime-proto also added RawNode. This is because this dag-pb
module leaves raw nodes out, since those are now in
go-ipld-prime/codec/raw.

The logic is just three lines, but it's still easier to not have to
copy-paste these between different projects.

A similar change to Encode and Decode function names came in
go-ipld-prime v0.9.0, so this keeps this library in sync.

Improved package docs a bit to talk about the new go-ipld-prime
LinkSystem, which is a bit clearer to refer to than before.

Target-of-opportunity fix some typos in docs.